How to make a crystal material?
Using volume rendering, you can emit light from inside a glass object. If you use a procedural gradient texture to control the strength, you get a nice effect:


If you bump up the IOR, it helps make it look more like a crystal:

Also see
Controlling volume density

Original answer:
You can make a glass object emit light on other objects without affecting the appearance of the object to the camera with a light path node:

If the incoming ray is a

Camera ray

Transmission ray

Glossy Ray


Then the glass shader is used. If it's not any of the above, then the emission shader is used. If the ray is hitting the back of a face, it always uses the glass shader.
If you want you could also add some emission which is visible to the camera, to make it appear more glowy:

If you want light to be emitted from inside the object, you can use the new volumetrics features: